This invention relates to a method of testing a vapour recovery system associated with a tank for a volatile liquid, such as a tank for holding petroleum spirit (hereinafter referred to simply as xe2x80x9cspiritxe2x80x9d) as installed at a fuel filling station for motor vehicles. The invention further relates to a method of testing a vapour recovery system of a tank farm, comprising a plurality of such spirit tanks, and a method of testing a tank installation.
Historically, a spirit tank at a fuel filling station had a simple vent-pipe, leading to atmosphere. This allowed vapour displaced from the tank during a refilling operation to be vented to atmosphere. Between deliveries, the vent-pipe allowed natural venting of the tank to take place as well as the ingress of air upon spirit being withdrawn from the tank, to be dispensed through a fuel delivery pump.
To overcome the problem of environmental pollution consequent upon this natural venting process, spirit tank farms have been, and are being, modified in various ways to operate on a current standard known as Stage 1B, where vapour displaced from a tank during a delivery process is returned to a road tanker delivering the spirit. A typical modification is to connect all of the individual spirit tank vent-pipes to a common manifold which has a single vent-pipe fitted with a pressure/vacuum valve (referred to hereinafter as a xe2x80x9cp/v valvexe2x80x9d), or sometimes a plurality of vent-pipes each fitted with a p/v valve. A p/v valve is normally closed but opens if the pressure in the manifold to which it is connected falls below a pre-set sub-atmospheric value, caused by dispensing spirit, or if the pressure rises above some other pre-set value above atmospheric.
When spirit is to be loaded into one or more of the tanks, the common manifold is connected to a vapour recovery system on the delivery road tanker, and the spirit vapour is drawn back into the tanker to be processed back to liquid spirit. In this way, much of the vapour previously released to atmosphere can be prevented, at the time of refilling the tanks of a filling station.
In the United Kingdom, at the present time it is a legal requirement to have all vapour recovery systems at retail petrol filling stations registered with the appropriate authority. Shortly it will be a requirement that such a system is tested to ensure that the system operates correctly, efficiently and safely with any leaks falling within the low limits set within European guidelines.
The present invention aims at providing a method of testing one or more tanks intended to hold spirit, firstly to ensure that the installation is suitable for the fitting of a vapour recovery system and secondly, once such a vapour recovery system has been installed, that the system is operating correctly, efficiently and safely, with no significant leaks that would prevent the recover system from operating correctly, efficiently and safely.
According to a first aspect of the present invention, there is provided a method of testing a volatile liquid tank installation having a fill-pipe projecting downwardly into the tank with the fill-pipe outlet below the normal minimum liquid level in the tank and the tank also having a vent-pipe, in which method one side of a shut-off valve is connected to the vent-pipe, a flow meter is connected to the other side of the shut-off valve, said valve is opened, liquid is supplied to the tank so as to increase the volume of liquid therein, and the out-flow of gas or wet vapour from the vent-pipe is monitored for substantial correlation to the volume of liquid admitted to the tank.
It will be appreciated that this method permits the testing of individual spirit and/or diesel tanks as installed for example at a fuel filling station, where those tanks are naturally vented as has been described above, prior to the fitting of a vapour recovery system such as that known in the UK as a Stage 1B system. By monitoring the volume out-flow of vapour from the vent-pipe for a substantial correlation with the volume in-flow into the tank, and preferably also the vapour flow rate and time of out-flow, for comparison with the time and in-flow of liquid fuel, there can be a reasonable assurance that there is no significant leak, permitting the escape to atmosphere of the vapour, from some other point in the installation. If the correlation falls outside expected limits, a leak, blockage or restriction may be suspected and a suitable investigation commenced.
Advantageously, a pressure gauge is arranged to sense the pressure in the vent-pipe, liquid is supplied to the fill-pipe so as to increase the volume of liquid in the tank, and the shut-off valve is operated to control the build-up of pressure in the vent-pipe up to some maximum value, consequent upon the displacement of vapour or wet vapour from the tank. At the completion of the delivery of liquid to the tank, the shut-off valve is closed and the subsequent decay of the pressure in the vent-pipe is monitored. By monitoring this decay, it is possible to determine whether there are leaks; and by taking into account the various relevant parameters (such as tank volume, ullage space, starting pressure and so on), then the seriousness of a suspected leak may be assessed.
Preferably, the testing is performed in the above orderxe2x80x94that is to say, the out-flow of vapour on supplying liquid to a tank is checked prior to testing for the decay in pressure allowed to build in the vent-pipe, when the valve is closed.
Even prior to performing the test described above, a preliminary step may be performed, in which the shut-off valve is closed and the pressure within the vent-pipe is monitored as liquid is drawn from the tank. Such drawing of liquid may be in the course of the filling of the tanks of motor vehicles and should create a negative pressure in the vent-pipe; this part of the test will also serve to ensure that there are no, or only minimal leaks.
Once, the individual tanks have been tested and found to comply within the permitted pre-set limits, the Stage 1B vapour recovery system may be installed. Then, that implementation may be tested for compliance and it is recommended that the installation is tested periodically for continuing compliance, typically once every twelve months.
According to a second aspect of this invention, therefore, there is provided a method of testing a vapour recovery system installed at a volatile liquid tank farm comprising a plurality of volatile liquid tanks each having an individual fill-pipe projecting downwardly into the tank with the fill-pipe outlet below the normal minimum liquid level in the tank and each tank having a vent-pipe coupled to a common manifold, in which method the common manifold is closed to atmosphere and one side of a shut-off valve is connected to the manifold, a flow meter is connected to the other side of the shut-off valve, flow meters are coupled to all but one of the fill-pipes, said shut-off valve is opened, liquid is supplied to the remaining fill-pipe so as to increase the volume of liquid in the associated tank, and the out-flow of gas of wet vapour from the common manifold is monitored for substantial correlation to the volume of liquid admitted to the tank associated with said remaining fill-pipe.
Preferably, a pressure gauge is arranged to sense the pressure in the manifold, liquid is supplied to said remaining fill-pipe so as to increase the volume of liquid in the associated tank, the shut-off valve is operated to control the build-up of pressure in the manifold consequent upon the displacement of gas or wet vapour from the tank, the shut-off valve is closed upon the completion of the supply of liquid to the tank, and the subsequent decay of the pressure in the manifold is monitored. In the alternative, the shut-off valve need not be closed until all tanks have been supplied with liquid, the decay then being measured for the whole system, at the very end of the testing procedure.
In a Stage 1B vapour recovery system, the common, manifold is fitted with one or more p/v valves. For such a case, the vacuum operation of the p/v valve should be tested before the performance of the testing method, by having the shut-off valve closed and checking for a negative pressure in the manifold. The pressure side of the p/v valve may be checked later, again with the shut-off valve closed, and allowing pressure to build until the p/v valve opensxe2x80x94which normally should be at 35 mbar, for the systems currently in use.
With a Stage 1B system, vapour displaced from a spirit tank by incoming fuel into that tank is recovered by being drawn back to the delivery tanker from the manifold, and this relies on the tanker generating a sub-atmospheric pressure to draw the vapour. For this purpose, a hose is arranged to connect the other side of the shut-off valve to a vapour recovery system (such as on a tanker), and a pressure gauge is arranged to sense pressure in that hose, to test for a sub-atmospheric pressure generated by the vapour recovery system, such as on the tanker.
For a multi-tank farm, the testing method should be repeated for each tank of the Farm, with liquid being supplied to the different tanks during each performance of the testing method. Repeated performance of the testing method may be effected by transferring a flow meter from one fill-pipe to another and then supplying the liquid to the pipe from which the flow meter was removed, and monitoring for out-flow of gas or wet vapour from the other fill-pipes.